FIGS. 1A and 1B illustrate a basic structure of a coil device 1. FIG. 1A is a perspective view illustrating the coil device 1. FIG. 1B is a cross-sectional view of FIG. 1A in the direction of view arrows a-a. That is, when a winding axis of the coil 4 is arranged in the vertical direction, if “front”, “rear”, “left” and “right” are defined as illustrated in FIG. 1A, FIG. 1B is a diagram of a cross-section of the coil device 1, as viewed from arrows a-a of FIG. 1A, extending in the vertical direction and the left-right direction. The coil device 1 includes an electronic component (hereinafter, also referred to as a coil component 10) such as a transformer, the transformer including: a well-known EE-shaped core 2, which has left and right side faces and in which two core members 2u and 2d having an E-shape as seen from front are arranged opposite in the vertical direction; and a coil 4 formed by winding conducting wires around a center leg 3 of the core 2. In addition, the lower face 11 of the core 2 is in contact with a heat-sinking board 5. As a result, heat generated by electrically conducting the coil 4 is guided to the heat-sinking board 5 through the core 2 so as to cool the coil component 10.
As for an electronic module such as a DC-to-DC converter composed of the coil device 1, miniaturization and increase of output are demanded. And, the increase of output of the coil device 1 directly leads increase of output of the electronic module. In addition, miniaturization of the coil device 1, whose footprint is larger than other electronic components, significantly contributes to miniaturization of the electronic module. However, miniaturization and increasing output of the coil device 1 makes it difficult to effectively dissipate the heat generated from the coil 4.
Specifically, the increase of output of the coil device 1 may be achieved by increasing an electric current flowing through the coil 4. However, if a large current exceeding a saturation magnetic flux density of the core 2 flows through the coil 4, a switching element used to drive the coil device 1 maybe broken down. In this regard, a gap for preventing a magnetic saturation (gap 20 in FIGS. 1A and 1B) is provided in the core 2 of the coil device 1. However, seeking the increase of output means requiring to obtain a large magnetic flux density by flowing a large current to a winding of the coil 4. This increases heat generated by the coil device 1 of higher output. In addition, the gap 20 which is an air layer having a low heat conductivity is indispensable for the increase of output of the coil device 1. For this reason, it is difficult to achieve both the increase of output and the improvement of heat dissipation efficiency in the coil device 1. In particular, the upper core member 2u is not in direct contact with the heat-sinking board 5, and the member 2u is not easily cooled down because a path to the heat-sinking board 5 from the center leg 3 around which the coil 4 serving as a heat source is wound is substantially split. Furthermore, the miniaturization of the coil device 1 reduces a contact area with the heat-sinking board 5. This makes more difficult to dissipate the heat. Naturally, the miniaturization of the coil device 1 decreases a heat generated in the core 2, and this makes it easier to increase a temperature under the same amount of heat. In addition, since the miniaturization of the coil device 1 decreases a surface area of the core 2 that exposes the atmosphere or a surface area that radiates the heat to the atmosphere, it is difficult to effectively discharge heat to the atmosphere. If the heat dissipation is insufficient, the coil device 1 may suffer from thermal runaway, and the coil device 1 may lost its function. Moreover, for the miniaturization of electronic modules, it is necessary to mounted the electronic components densely around the coil device 1, and this may cause thermal breakdown of electronic components around the coil device 1. Naturally, a cooler (such as a fan) for suppressing a rise of the temperature inevitably increases the size of the electronic module.
In this regard, a technique has been proposed in Patent Literature 1.